Generally with a capacity increase of storage devices, data protection may have become worse. The reason may be that the capacity increase of the storage device (e.g., a disk) causes an increasingly longer time period required for reconstructing the storage device upon failure. The longer reconstruction time period means higher risks for data on the storage device such as Redundant Array of Independent Disks (RAID).
Generally during a reconstruction time period, data reliability may be reduced dramatically due to loss of redundancy information. As an example, consider RAID-6 to see change of data reliability in case of disk failure. Referring to FIG. 1, it can be seen that RAID-6 can tolerate 2 disks failure simultaneously, thanks to existence of redundancy information when no disk fails. Once one disk fails, reconstruction need to be started and RAID comes into a degraded state. However, for RAID-6 there may still be redundancy information available, so even g in a degraded state, it may still be able to tolerate failure of one disk. Unfortunately, during a reconstruction period, if another disk also fails, that will get RAID into an inferior state, in which the RAID cannot tolerate any more disk failure. A graver situation may be that the RAID may stay in this inferior/poorer state for a long time until the first failure disk completes its reconstruction. For this reason, there may be a high risk of data loss in such a state, and it therefore becomes hard to guarantee reliability of data on the RAID. FIG. 1 schematically illustrates the following three states of a RAID-6:
1) The first state can be called a robust state. In this state, RAID can tolerate simultaneous failure of two disks. If none of its disks fails, the RAID should be in this state.
2) The second state can be called a safe state. In this state, RAID can tolerate failure of one disk.
3) The third state can be called a vulnerable state. In this state, RAID cannot tolerate failure of any disk any more.
Both the second and the third states are considered as a degraded state for RAID, and the time period of remaining in a vulnerable state without any redundancy protection may be too long. Similar problems may also exist with other storage devices. With capacity increase of storage devices, reconstruction time becomes longer and longer, so does the time duration of staying in the vulnerable state, and makes data reliability poorer.